Moisture content is a key production parameter in many processes. Materials of interest are wide ranging and include food-stuffs, chemicals, mineral ores, mineral concentrates, coal, oil and gas. For some time it has been recognised that electromagnetic wave interaction with the material of interest may provide a method for moisture determination. Microwave-based methods rely on the observed high correlation between moisture content and either one or more of the parameters of wave phase shift, reflectance or attenuation. This correlation exists for a wide range of materials. Fundamentally the correlation occurs because unbounded water exhibits a dielectric constant with a very large magnitude compared to the material in which it is entrained. The effective dielectric constant may be derived from the dielectric tensor, which is the most general physical quantity that describes wave dispersion in all types of linear media, both magnetic and non-magnetic. In general the non-magnetic material response is dependent on both real and imaginary parts of the effective dielectric constant. However, the phase shift and reflectivity is usually mostly affected by the real part for typical materials of interest, while attenuation is more dependent on both real and imaginary parts. Similar considerations apply for magnetic materials, so long as the magnetic response remains somewhat weaker than the dielectric response imparted by the presence of free moisture.
Microwave methods may generally be divided between different classes of measurement. For example, measurements may employ the apparatus of tuned microwave resonant cavities. Another class of measurement involves free space measurements. In free space measurements-radiating structures (antennas) launch electromagnetic waves which are transmitted without the use of any guiding structure towards the material of interest. Free space methods have the general advantage that the apparatus does not constrain the flow of material in any way, which may be important in industrial processes. Free space measurements themselves may be generally subdivided between transmission and reflection measurements.
In reflection measurements the wave reflected off an air-material interface is measured. This wave may be received by the same structure that is used to transmit the wave, or with alternative receiving antennas. The amplitude and phase of the reflected wave is correlated in some way to the moisture content.
In transmission measurements, the waves are allowed to propagate through the material of interest from one side and are measured with a separate receiving antenna on the other side of the material layer. Either or both the phase shift and attenuation of the wave through the material may serve as the basic variable used to estimate moisture content, although phase shift generally demonstrates the better correlation to moisture content. However, phase shift and attenuation are also generally dependent on both the density and thickness of the material. Specifically, phase shift varies most linearly with the quantity defined by material mass per unit area (MPUA) presented to the transmitted wave. Because of inevitable variation of this quantity in most applications, an auxiliary or normalising parameter approximating MPUA is usually required to compensate for these variations. For example, MPUA may be approximated by either of the parameters of material height or mass loading. The auxiliary parameter is used to provide a weighting parameter to normalise the wave phase shift before moisture is inferred from the measurement. Where significant non-linearities exist between the weighting parameter and MPUA this will increase error in the estimated moisture content.
An important detail of free space transmission technology is the magnitude of phase shift imparted by the material. Often the phase shift is greater then 360 degrees. Since a single measurement of phase can only define the phase shift in the 0–360 degree range, measurements that rely on a single transmission frequency are often limited to materials that do not change markedly in presentation or moisture content. This fact limits the utility of single frequency measurements. Presently the state of the art in transmission measurements is to employ multi-frequency methods. There are several classes of multi-frequency method. One class employs two (dual) discrete frequencies separated by a modest frequency range (high frequency approximately 1.05 to 2 times the lower frequency). Another class employs a multitude of discrete frequencies or a continuum of frequencies within a frequency band; these may generally be regarded as “swept” frequency methods. Yet another class may employ electromagnetic wave pulses (in this case the finite length of the pulse implicitly defines the frequency band that is used by the generation of sidebands). In all cases the information carried in extra frequency components is used to resolve the absolute phase shift beyond 360 degrees.
Error in transmission measurements may derive from a number of sources. One source of error is the reception of spurious waves with phase that may be unrelated or weakly related to the moisture in the material. Such spurious waves include waves reaching the receiver that have not passed through the material (or only a small section of material), and waves which pass several times through the material by way of multiple reflections. These spurious waves act to distort the measured phase in all types of transmission measurements. Another source of error may be from limitations in the receiver itself. For example the receiver may lose sensitivity at high attenuation, resulting in the measurement of phase that is not a true representation of the actual received wave phase.
It would be desirable to provide an improved technique for estimating moisture content.